The invention relates to a pick-up for the inductive production of a measurement signal which reproduces the position of first and second mutually movable bodies.
Position pick-ups or sensors for that purpose serve to generate an electrical signal, by means of which it is possible to monitor or trace and measure a continuously or intermittently occurring relative movement between first and second bodies, in such a way that at any moment information is available about the instantaneous position of one of the bodies relative to the other.
One alternative configuration of such a pick-up may be a linear pick-up or sensor with which for example the movement and/or instantaneous position of a machine carriage which is displaceable relative to a machine frame structure is to be detected and controlled with a high degree of precision. That situation requires the constant production of a signal which provides information about the instantaneous position of the carriage, even when the carriage is moving at high speed.
Another pick-up construction is represented by a rotary pick-up or sensor for measuring the instantaneous angular position of a rotating body, for example the rotor of an electric motor relative to the stator, or the rotary angle between two bodies which are rotatable relative to each other, for example the azimuth or vertical angle of the telescope of a theodolite.
In a similar fashion, a rotary pick-up or sensor can be used to measure the angular positions or speeds of rotation of motor vehicle wheels or the instantaneous angular position of a carburettor butterfly valve.
A linear pick-up can be found in German patent specifications Nos 25 11 683 and 26 17 624. Those pick-ups each include a ferromagnetic flux guide means having two rectangular elongate flat plates which are arranged in mutually parallel relationship in such a way that they enclose an air gap between their flat sides. At one of the two short sides of the rectangular configuration, those plates are connected by a limb portion extending perpendicularly to the planes of the plates, in such a way as to define a U-shaped cross-section. The limb portion extends through an exciter coil which is fed with alternating current to generate a magnetic flux which can follow an annularly closed path, across the air gap, with a substantially homogeneous magnetic field being produced in the air gap.
That flux guide means is connected to one of the two mutually movable bodies while coupled to the other body is a measurement coil arrangement which is in the form of a printed circuit and which has two measurement coils of which each includes a plurality of windings, each embracing a respective surface element.
The surface elements which are formed in that way are of different sizes and are arranged in interleaved relationship with each other. All in all, that arrangement gives an elongated measurement coil configuration which extends in the direction of the movement to be monitored, and defines the maximum width thereof. The carrier board of the measurement coils is disposed between the two plates of the flux guide means and parallel to the plates. Projection, in the direction of the magnetic flux, of the mutually oppositely coincidentally disposed wall surfaces of the air gap between the plates, on to the surface of the carrier board and thus the measurement coil, defines there a substantially rectangular transit surface, the longitudinal direction of which extends perpendicularly to the direction of movement transversely over the entire measurement coil arrangement, and the width of which, in the direction of movement, is substantially shorter than the maximum width of movement. The main part of the magnetic flux which crosses over in the gap passes through the measurement coil arrangement in the above-mentioned transit surface, although considerable leakage flux components may also occur which extend outside that purely geometrically defined transit surface and which at least in part also pass through the surface elements of the measurement coil arrangement.
If one of the two bodies to be monitored moves relative to the other body, then the transit surface is displaced over the surface elements of the measurement coil arrangement whereby the magnetic flux passing through the individual windings changes so that the measurement coils respectively produce an electrical output signal of variable amplitude. The output ac voltage signals produced by the measurement coils are rectified to form a measurement signal, the magnitude of which is characteristic in respect of the instantaneous position which one of the bodies occupies relative to the other. In order to produce a measurement signal which is symmetrical relative to the zero potential associated with the central position, the measurement coils are of such a configuration and are so arranged in mirror image relationship with each, other that regions of the transit surface which, upon movement of the bodies, leave the condition of overlap with the one measurement coil, pass into a condition of overlap with the other measurement coil and vice-versa; in that situation, the difference in the voltages which are supplied by the measurement coils is to follow a linear configuration as accurately as possible, over as large a part as possible of the range of the movement to be monitored. In the case of configurations in which another output signal characteristic is to be achieved, it is desirable in a corresponding fashion for the characteristic line which is theoretically predetermined by the selected configuration to be maintained as accurately as possible over the entire range of movement.
In addition, the step of forming the difference from the voltages produced by the measurement coil signals is intended to ensure eliminating additive interference values from the measurement signal.
However it has only been possible hitherto to attain both those aims to an inadequate degree. Thus it can be seen for example from German patent specification No 25 11 683 that the linear configuration of the measurement signal characteristic, which that arrangement seeks to achieve, terminates not in a sharp point but a rounded point, when approaching the two limit positions. It is also found in practice that the characteristic of such an arrangement also extends not linearly but distorted in an S-shaped form, in the region of the passage through zero. Although such non-linearities can be partially compensated by means of the electronic circuitry disposed on the output side of the pick-up or sensor, that however involves additional structure and thus increased costs.
It is further found that a measurement signal which is obtained by the above-indicated difference-forming step still includes major interference signal components which cannot be properly eliminated by the difference-forming operation.